Interestingly, the Kondo hybridization does not appear significant at first
glance in band structure calculations [1]}. It was later realized
that nickelates may host a special interstitial-\(s\)  orbital for conduction
electrons that have substantially stronger hybridization than previously
thought [2]}. Resonant inelastic X-ray scattering (RIXS)
measurements [3]}, [4]} confirmed the presence of
hybridization between Ni \(3d_{x^{2}-y^{2}}\)  and Nd \(5d\)  orbitals. At zero
temperature, the self-doping effect and the Kondo coupling produce
low-energy doublon (Kondo singlet) and holon excitations on the nickel
spin-1/2 background. Because of the larger charge transfer energy,
nickelates have a reduced superexchange interaction between Ni\(^{1+}\)  spins
by almost an order of magnitude than cuprates. Raman scattering measurements
confirmed this expectation and estimated \(J\approx 25\,\) meV in bulk NdNiO\(_{2}\)  [5]}. The Kondo coupling may therefore suppress the AF
long-range order and cause a phase transition to a paramagnetic metal [6]}. The parent  or underdoped compounds may therefore be
viewed as a Kondo semimetal (KS). For large hole doping, the Ni-3\(d\) 
electrons become more itinerant and the Kondo effect breaks down, followed
by an abrupt change of the charge carriers. Indeed, a sign change of the
Hall coefficient has been reported in experiment [7]}, [8]}.
